[vpm] __match determines match semantics; virtualization

determine match strategy by typing `__match`
factored out the interface to generate code in this monad, cleaned up codegen a bit

no longer solving a context bound to determine the match strategy and the monad's type constructor
it's too expensive

don't consider implicits looking for __match
implicit search causes HUGE slowdowns -- now the overhead is about 4% compared to just assuming there's no __match in scope

to support virtualization&staging, we use the type of `__match.one` as the prototype for how to wrap "pure" types and types "in the monad"
pure types T are wrapped as P[T], and T goes into the monad as M[T], if one is defined as:

  def one[T](x: P[T]): M[T]

for staging, P will typically be the Rep type constructor, and type M[T] = Rep[Option[T]]

furthermore, naive codegen no longer supplies type information -- type inference will have to work it out
optimized codegen still does, of course, and that's enough since we only bootstrap that way

TODO: improve the test (currently the condition is not represented)

Author: Adriaan Moors